1. Field of the Invention
The present invention relates to a sheet transporting apparatus for guiding a sheet to an aftertreatment apparatus and having a function of correcting a curl of the sheet, and to an image forming apparatus provided with the sheet transporting apparatus.
2. Description of Related Art
An image forming apparatus (such as a copying machine, a printer and the like) employing an electrophotographic process transfers a toner image formed on a photosensitive drum (or an electrophotographic photosensitive member) being an image bearing member to a sheet, and the image forming apparatus passes the sheet, on which the toner image has been transferred, through a fixing device. The image forming apparatus thereby fixes the toner image on the surface of the sheet to be a permanent image. Incidentally, silicone oil is generally applied on the surface of the sheet that is passing through the fixing device in order to prevent the toner of the toner image and the sheet from adhering to a fixing roller.
Moreover, an image forming apparatus equipped with a sheet aftertreatment apparatus for sorting sheets, on which images have been fixed, with a sorter, a finisher or the like is known. Besides, an image forming apparatus equipped with a sheet transporting apparatus (hereinafter referred to as a xe2x80x9cbuffer unitxe2x80x9d) for transporting sheets delivered from the main body of the image forming apparatus to the sheet aftertreatment apparatus is known.
An image forming apparatus employing the electrophotographic process has a full color mode for superimposing four color toners on a sheet as in a full color machine, a monochromatic mode for forming an image only by one color toner, and a two-color and a three-color modes for superimposing two and three color toners on a sheet respectively.
Generally speaking, when a sheet bearing a toner image on one side thereof has been passed through pressure and heating rollers for the fixation of the toner image, the sheet curls to bend to the side of the image surface thereof on which the toner image is borne.
However, when such a curled sheet, which has passed through the fixing device, is contained in a sheet aftertreatment apparatus such as the sorter, the finisher or the like, a delivery tray or the like, a faulty sheet transport such as a sheet jam may often occur, and there is a possibility that the functions of the sheet aftertreatment apparatus such as the sorter, the finisher or the like are not fully performed.
Accordingly, it is generally performed to dispose the buffer unit having a function of correcting the curl formed in a sheet between (the fixing device of) the main body of the image forming apparatus and the sheet aftertreatment apparatus such as the sorter, the finisher or the like or the delivery tray.
Incidentally, the buffer unit may be integrally provided to an image forming apparatus to be one body. Alternatively, the buffer unit may be discretely provided between the main body of the image forming apparatus and the sheet aftertreatment apparatus as a separate body.
Next, FIG. 14 and FIG. 15 are referred while a related art sheet transporting apparatus and a related art image forming apparatus are described more minutely. Incidentally, for the sake of the convenience of descriptions, the same component as those disclosed in the drawings to be referred in the descriptions of a sheet transporting apparatus and image forming apparatuses according to the embodiments of the present invention that will be described later are designated by the same reference numerals or signs, and the descriptions about the same components are suitably omitted.
As shown in FIG. 14, the related art image forming apparatus includes a main body 400 thereof for forming an image on a sheet, a sheet aftertreatment apparatus (hereinafter referred to as a xe2x80x9csorterxe2x80x9d) 200 having a function of sorting sheets on which images have already been formed, and a sheet transporting apparatus (hereinafter referred to as a xe2x80x9cbuffer unitxe2x80x9d) 100 disposed between the main body 400 and the sorter 200.
Incidentally, the shown image forming apparatus has a configuration in which the main body 400, the buffer unit 100 and the sorter 200 are separately disposed. But an image forming apparatus may be provided with these components formed in one integral body.
The buffer unit 100 has a function of switching the transportation state of a sheet fed from the main body 400 between the transportation of the sheet without the reversing of the surface thereof and the transportation of the sheet with the reversing of the surface thereof.
Moreover, the buffer unit 100 is provided with correcting means 104 and 110 for correcting the curl formed in a transported sheet.
The correcting means 104 corrects an up-curl, or upward bending of a sheet, and the correcting means 110 corrects a down-curl, or downward bending of a sheet.
And, because a sheet ordinarily curls to bend toward one side of the two-side copied sheet in which the image density of a formed image is higher than that on the other side of the sheet, either of the correcting means 104 and 110 is operated on the basis of the information concerning the image densities.
Moreover, the sorter 200 is equipped with a non-sort bin B0 at the uppermost position and a plurality of sort bins B1, B2 . . . and B20 at lower positions. The sorter 200 has a function of delivering transported sheets while performing the sorting of the sheets such as the collating and the grouping of the sheets.
As described above, conventionally, when the correcting means 104 and 110 as curl correcting means are provided in the buffer unit 100, the correcting means 104 and 110 are controlled on the basis of the difference between the image densities of images borne on both sides of a sheet S to be fed into a sheet aftertreatment apparatus such as the sorter 200 regardless of the processing method of the sorter 200.
However, such a curl correction method being performed on the basis of the difference between the image densities on both sides has a problem of the occurrence of a paper jam (hereinafter simply referred to as a xe2x80x9cjamxe2x80x9d) owing to the waving of sheets S (see, for example, the sheets S stacked on the sort bin B7 shown in FIG. 15) when the sheets S are delivered on the sort bins B1-B20 of the sorter 200 in a group mode or a stack mode.
The problem is described more minutely in the following.
In the group mode or in the stack mode, the sheets S are delivered on the sort bins B1-B20.
Hereupon, the curl correction of the sheets S (the up-curl correction of the sheets S by the correcting means 104 in the example in FIG. 15) is performed in the buffer unit 100, and the sheets S are transported through a sort path 205 and delivered by delivery rollers 206.
Immediately after the correction of a sheet S, the sheet S is slightly curled reversely to the curl in the sheet before the correction. That is, the sheet S in the example shown in FIG. 15 is slightly down-curled because the sheet S has suffered from the up-curl correction.
Because such sheets S are continuously delivered to the sort bin, a successive sheet S is delivered before the curl (or the up-curl) of a previous sheet S, which has been previously delivered, owing to the toner of an image on the sheet S has grown up. Consequently, the down-curled sheets S continue to be stacked in the sort bin. Then, the height of the stacked delivered sheets S becomes high. Consequently, as shown in FIG. 15, a part of the sheets S stacked on the sort bin B7 is brought into contact with the lower surface of the sort bin B6 positioned immediately above the sort bin B7. Thereby, there is the case where the contact prevents the delivery of a sheet S to be delivered next onto the sort bin B7 to cause a sheet jam.
Moreover, when the sheets S are continuously delivered on the stacked sheets S having still maintained curls formed by the curl correcting means even if the sheets S are delivered on a tray having a sufficient space above the tray like an ordinary delivery tray, a sort tray or the like other than the sort bins, there is a case where a sheet jam may occur due to the interference between the stacked sheets S on the tray and a sheet S that is being delivered.
The present invention was made for resolving the problems of the aforesaid related art. An object of the present invention is to provide a sheet transporting apparatus and an image forming apparatus, both being capable of preventing the occurrence of a sheet jam due to a sheet delivered in an aftertreatment apparatus.
For attaining the object, a sheet transporting apparatus of the present invention comprises: a transporting path for guiding sheets to an aftertreatment apparatus having a function of sorting and delivering the sheets to a plurality of bins; and correcting means for correcting a curl formed in a sheet transported through the transporting path, wherein, when the aftertreatment apparatus continuously delivers the sheets to sort bins other than a non-sort bin at an uppermost position among the plurality of bins, the correcting means does not perform a correcting operation, or the correcting means performs the correcting operation while lowering a degree of correction than that at a time of delivering the sheets to the non-sort bin.
Moreover, a sheet transporting apparatus of the present invention may comprise: a transporting path for guiding sheets to an aftertreatment apparatus having a function of sorting and delivering the sheets to a plurality of bins; and correcting means for correcting a curl formed in a sheet transported through the transporting path, wherein, when a group mode for delivering the sheets continuously to a predetermined sort bin other than a non-sort bin at an uppermost position among the plurality of bins is selected, the correcting means does not perform a correcting operation, or the correcting means performs the correcting operation while lowering a degree of correction than that at a time of delivering the sheets to the non-sort bin.
Moreover, a sheet transporting apparatus of the present invention may comprise: a transporting path for guiding sheets to an aftertreatment apparatus having a function of sorting and delivering the sheets to a plurality of bins; and correcting means for correcting a curl formed in a sheet transported through the transporting path, wherein, when a stack mode for delivering the sheets to sort bins other than a non-sort bin at an uppermost position among the plurality of bins if a number of the sheets to be delivered exceeds a number of sheets stackable on the non-sort bin is selected, the correcting means does not perform a correcting operation, or the correcting means performs the correcting operation while lowering a degree of correction than that at a time of delivering the sheets to the non-sort bin.
It is preferable that a sheet transporting apparatus further comprises surface reverse means for reversing a front and back sides of a sheet transported from an upstream side to guide the reversed sheet to the aftertreatment apparatus.
Moreover, an image forming apparatus of the preset invention may comprise: image forming means for forming an image on a transported sheet; a sheet transporting apparatus according to any one of the aforesaid aspects of the invention for transporting the sheet on which the image has been formed by the image forming means to a further downstream side; and an aftertreatment apparatus having a function of sorting and delivering the sheets transported by the sheet transporting apparatus to a plurality of bins.
It is preferable that an image forming apparatus comprises four image forming means for forming images having different colors from each other, the four image forming means being arranged in tandem, to enable formation of a full color image.
It is preferable that, when a number of sheets stacked on a sort bin other than a non-sort bin at an uppermost position among a plurality of bins installed in the aftertreatment apparatus reaches a maximum stackable number of sheets, or when a delivery of the sheets ceased regardless of the number of the stacked sheets, the stacked sheets on the sort bin are sandwiched, or clenched between the sort bin and a sort bin immediately above the sort bin.